The Slingshot Argument

(1) The fact that p

(2) The fact that a = [the object x such that (x = a and p is true)]

(3) The fact that a = [the object x such that (x = a and q is true)]

(4) The fact that q

(1) = (2) and (3) = (4) by logical equivalence

(2) = (3) because the right-hand sides of the identity refer to the very same thing, namely object a

Therefore, (1) = (4)